Note: Descriptions are shown in the official language in which they were submitted.
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BRUSHLESS DC MOTOR
Field of the Invention
The invention relates to Brushless DC Motors (BLDC).
Background of the Invention
In its simplest conceptual form (shown in Fig. 1), a BLDC motor consists
of a permanent magnet 1 (the rotor) which is free to rotate around its axis
of symmetry, surrounded by an arrangement of at least three fixed
electromagnets (the stator) consisting of solenoids windings 2, positioned
at 1200 relative to each other around the rotor axis. Each solenoid is
energized by applying to it a DC (Direct Current) voltage, by means of a
set of electronic switches 3, operated with timing and polarity determined
by a switch-control algorithm.
If the electromagnets are energized with the proper timing and polarity,
they generate a magnetic field with the proper strength and direction
relative to the S-N axis direction of rotor magnet 1, and this magnetic field
produces a torque on the permanent magnet causing the rotor to turn. The
algorithm determines the required operating sequence of the switches at
any given moment, according to the actual angular position of the rotor,
said position being determined by means of one or more sensors, usually of
the Hall type (indicated in the figure by numeral 4), which sense the
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magnetic field of the rotor. The operation of the motor, which is housed in
a housing 5, is controlled by a controller 6.
In the simple conceptual form of Fig. 1, it is enough to properly energize
two magnets at a time in order to generate a rotating magnetic field of
arbitrary direction that will keep the rotor turning. In practice, in order to
obtain a continuous smooth torque value, BLDC motors are implemented
using many windings for the stator, and several magnets with alternating
N-S poles for the rotor.
There are two basic BLDC motor architectures known in the art: the
inner rotor architecture (Fig. 2a), where the stator windings surround
the rotor and are affixed to the motor's housing, and the outer rotor
architecture (Fig. 2b), where the stator solenoids are affixed in the core
of the motor, and are surrounded by the rotor magnets. In the prior art
implementations, BLDC motors suffer from the following drawback: for a
fixed supply voltage, as the motor speed increases there is a decrease in
the torque that the motor can provide. This undesirable effect is the result
of the generation of a parasitic voltage, known as the back EMF
(Electromotive Force) voltage.
The back EMF is a voltage generated in the stator much in the same way
an electric generator works, because there is relative motion between
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the solenoids of the stator and the magnetic field created by the
permanent magnets of the rotor. The magnetic field lines created by the
permanent magnets rotate along with the rotor. Thus, the projection (in
the direction of the solenoid axis) of the magnetic field lines entering the
cross-sectional area of each of the energized solenoids, changes with time.
This projection of field lines sums up to a quantity referred to as "the
magnetic flux" through the solenoid. By Lenz's law of induction, a
changing magnetic flux produces an induced voltage in the solenoids (in
this respect, the motor acts like a generator). The value of this induced
voltage increases proportionally to the rate by which the flux changes, and
therefore it increases with increasing rotating speed of the motor, and its
polarity opposes the original voltage externally applied by the supply. As
a result, the overall effective voltage applied to each energized solenoid of
the stator decreases with increasing angular velocity of the rotor (the
overall voltage equals the constant external supply voltage, less the back
EMF induced). Due to the decrease in the overall voltage applied, the
current flowing into the solenoids of the stator decreases too, which
ultimately results in a reduction of the torque provided by the motor.
Therefore, the maximal torque that the motor can deliver drops as the
rotating speed increases. In order to increase back to torque at high speed,
one needs to increase the supply voltage, which in many instances cannot
be done.
4
Another adverse side effect of back EMF generation is that, for a fixed supply
voltage, the
current flowing in the solenoids is higher at lower rotational speed, because
then the back
EMF is lower and the overall voltage applied to the solenoids is higher. It
follows that at
start (when there is no motion, and therefore there is no flux change and no
back EMF) the
motor drives the highest current. Since the supply voltage is significantly
higher than the
overall voltage applied to the solenoids at final speed, then, at motion start
one gets peaks
of current that are significantly higher than the steady-state working
current. Such
undesirable over-current peaks may even lead to solenoid damage or power
supply
overload, and sometime must be dealt with, by means of added protective
devices, or by
an overkilled design of current handling capability.
Summary of the Invention
Certain exemplary embodiments can provide a brushless DC motor capable of
generating
a substantially constant torque regardless of an angular velocity of its
rotor, comprising a)
a circular rotor comprising a plurality of circumferentially separated
permanent magnets
and a plurality of spacers made of high magnetic permeability material, each
of said spacers
being interposed between two of said permanent magnets to reduce variations in
axial
magnetic flux; and b) a plurality of circumferentially spaced solenoids
provided each
around a static solenoid housing, wherein said solenoid housing is structured
with a void
portion through which said plurality of magnets can pass when said rotor
rotates around its
axis, wherein motion of the magnets with respect to the solenoids is quasi-
linear in the
direction of a solenoid axis, wherein said plurality of solenoids is energized
by applying a
DC voltage in a timing sequence and alternating polarity to ensure that it
will interact with
only one of said permanent magnets at any given time, thereby minimizing back
electromotive force (back EMF).
Certain exemplary embodiments can provide a method for operating a brushless
DC motor,
comprising the steps of causing a plurality of permanent magnets to move with
respect to
a plurality of solenoids in a quasi-linear motion in the direction of the axis
of the solenoid,
and energizing said plurality of solenoids by a timing sequence that ensures
that each of
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said solenoids will interact with only one of said permanent magnets at any
given time,
thereby reducing back EMF.
Certain exemplary embodiments can provide a method for operating a brushless
DC motor,
comprising the steps of: a) providing on a circular structure, a plurality of
circumferentially
separated permanent magnets and a plurality of spacers made of high magnetic
permeability material arranged such that each of said spacers is interposed
between two of
said permanent magnets; b) providing a plurality of solenoids around a static
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solenoid housing, wherein said solenoid housing is structured with a void
portion through which said plurality of magnets and said plurality of
spacers can pass when the circular structure rotates around its axis; and
c) energizing each of said plurality of solenoids by a timing sequence that
causes said circular structure to rotate, while causing magnetic flux
through one of said solenoids as a result of an interaction with one of said
plurality of magnets passing therethrough at a given time, by virtue of a
constraining action provided by the spacer adjacent to said interacting
magnet, to be substantially constant along a circumferential length of said
adjacent spacer, thereby minimizing back EMF.
Other embodiments relate to a brushless DC motor, comprising a plurality
of magnets positioned at a distance from one another on a circular
structure, and a plurality of solenoids provided each around a static
solenoid housing, wherein said solenoid housing is structured with a void
portion through which said plurality of magnets can pass when the
circular structure comprising said plurality of magnets rotates around its
axis. The motion of the magnets with respect to the solenoids is quasi
linear in the direction of the axis of the solenoid. The term "quasi-linear"
is
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mean to indicate that when the magnet enters the housing of the solenoid,
its movement is almost linear with respect to the axis of the solenoid. Of
course, since the magnet is positioned on a circular path, the motion
cannot be fully linear, and hence the term "quasi-linear" is employed.
When spacers are provided between adjacent permanent magnets, they
should be made of high permeability material.
As will be apparent to the skilled person, different numbers of permanent
magnets and of solenoids can be provided, depending on the specific set-up
of the motor. According to one embodiment of the invention the number of
solenoids equals the number of permanent magnets and according to
another embodiment of the invention the number of solenoids can be
greater or smaller than the number of permanent magnets.
The brushless motor of the invention should be provided with one or more
sensors suitable to determine the position of permanent magnets relative
to solenoids. A controller should further be provided, suitable to allow the
supply of current to solenoid in response to a determination by one or
more sensors regarding the position of magnets relative to solenoids.
In one embodiment of the invention the permanent magnets and the high-
permeability material located between them, alone or together with one or
more structuring ring, form of the rotor of the motor, which is
mechanically connected to power-transmitting means, e.g., by a toothed
element.
6
In another aspect the invention relates to a method for operating a brushless
DC motor,
comprising the steps of: a) providing on a circular structure, a plurality of
circumferentially
separated permanent magnets and a plurality of spacers made of high magnetic
permeability material arranged such that each of said spacers is interposed
between two of
said permanent magnets; b) providing a plurality of solenoids around a static
solenoid
housing, wherein said solenoid housing is structured with a void portion
through which
said plurality of magnets and said plurality of spacers can pass when the
circular structure
rotates around its axis; and c) energizing each of said plurality of solenoids
by applying a
DC voltage in a timing sequence and alternating polarity that causes said
circular structure
to rotate, while causing magnetic flux through one of said solenoids as a
result of an
interaction with one of said plurality of magnets passing therethrough at a
given time, to
be substantially constant along a circumferential length of said adjacent
spacer, thereby
minimizing back EMF.
Brief Description of the Drawings
In the drawings:
Fig. 1 schematically shows a prior art motor;
Fig. 2 (a) and (b) shows two prior art architectures for a brushless motor;
Fig. 3 schematically shows the architecture of a motor according to one
embodiment of the invention;
Fig. 4 illustrates the field generated by two adjacent permanent magnets;
Fig. 5 is a schematic representation of a rotor according to one embodiment of
the
invention;
Fig. 6 illustrates a motor according to one illustrative embodiment of the
invention,
in assembled state;
Fig. 7 is a top view of the motor of Fig. 6;
Fig. 8 is a side view of the motor of Fig. 6;
Fig. 9 illustrates the movement of the motor of Fig. 6;
Fig. 10 illustrates the ring elements that are associated with the rotor of
Fig. 5 in the
motor of Fig. 6;
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Fig. 11 is an exploded view of the motor of Fig. 6;
Fig. 12 (b) and (c) are cross-sections of the central part (a) of the
motor of Fig. 6;
Fig. 13 illustrates the connection of the permanent magnets of the
rotor of Fig. 5 on the bottom ring of Fig. 10;
Fig. 14 illustrates the assembly of the solenoid houses on the rotor
assembly;
Fig. 15 illustrates an alternative rotor architecture; and
Fig. 16 illustrates the position of the poles of the magnets inside the
solenoid, for minimizing the EMF.
Detailed Description of the Invention
The invention is concerned with a novel type of BLDC motor architecture,
which leads to a major reduction of back EMF levels, thus yielding a motor
capable of providing a constant torque value regardless of the angular
velocity of the rotor. As a side benefit of the reduced back EMF, the overall
applied voltage is nearly constant, and therefore no over-current peaks at
start will occur.
The cause of back EMF generation in prior-art BLDC, is the change in
magnetic flux through the solenoids of the stator. This flux change is due
to the spinning of the rotor, which produces both changes in the magnetic
field strength present within the core of the solenoid (as a magnet of the
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rotor approaches a solenoid or moves away from it), as well as changes in
the direction of the field lines with respect to the axis of the solenoids (a
change in the component of the magnetic field crossing the core of the
solenoid parallel to its axis produces a change in magnetic flux through it).
During the circular movement of the magnets of the rotor, there is a
change both in the distance between magnets and solenoids and in the
direction of the magnetic field lines relative to the solenoids axis, which
both result in flux changes through the solenoids. The architecture subject
of the invention reduces the back EMF effect by reducing the above-
mentioned flux changes.
The motor architecture according to an embodiment of the invention is
schematically shown in Fig. 3, for the purpose of explaining the principle
by which it operates. The stator architecture consists of a number of air-
core solenoids 32 affixed to the motor housing, and whose axis of
symmetry is aligned along a circular path 35. The basic rotor architecture
consists of a number of permanent magnets 31 whose S-N axis is aligned
with alternate polarity along the same circular path as the stator. The
magnets may either be connected to each other with a high-permeability
material to form a continuous circular ring as shown in Fig. 5, or may be
mounted onto a circular flat basis 35 and the space between them may be
left open to air as in Fig. 3. The number of magnets may be larger, equal,
or smaller than the number of the solenoids.
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The rotor is supported by rotating mechanical bearings (not shown) and is
free to rotate around the center of its circular shape while passing inside
the core of the solenoids of the stator as shown in Fig. 6. The solenoids
are electrically connected to the DC supply through a system of switches
33, preferably, but not limitatively, of the electronic type, which
determines, at each instant, the polarity and the level of the voltage
applied to each solenoid in the stator. The switches are controlled by an
apparatus, preferably a microcontroller 36 with associated software, which
determines at each instant the DC polarity applied to each solenoid (e.g.,
by inverting the DC connection to it), as well as the average DC level (e.g.,
by applying the DC supply voltage using Pulse Width Modulation (PWM)).
The angular position of the rotor at each instant is detected by a system of
sensors 34 (e.g., optical sensors or Hall-effect sensors). The sensor output
is fed to the controller, which operates the switches according to the status
of the rotor (i.e. angular position, speed and acceleration).
When a solenoid of the stator is energized, the nearby magnets of the rotor
move along the circular path of the stator. The magnet is either pulled-in
towards the solenoid core, or pushed-out from it, depending on the polarity
of the switch associated with said solenoid, which determines the direction
of flow of the current in the windings, and on the orientation of the
magnets (N-N or S-S). In turn, the status of said switch is determined at
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each time by the controller, based on the angular position of the rotor
detected by the sensors. Under the proper simultaneous operating
sequence of the overall system of switches, it is possible to obtain a
continuous smooth rotation of the rotor in either spinning direction. The
motion of the rotor is then transferred to the load by means of a
mechanical gear 63 coupled to the rotor ring, as shown in Fig. 6.
Without wishing to be bound by any specific theory, the inventors believe
that a possible mechanism that leads to the reduction of the back EMF, as
a result of the novel motor architecture of the invention, may be as
described below. As it may be readily appreciated from Fig. 3 and the
previous description, the motion of the magnets of the rotor with respect to
the solenoids, is quasi-linear, namely, in direction of the axis of the
solenoid. This is in contrast to the prior-art architecture in which the
motion of magnets of the rotor is transversal, namely in a direction
perpendicular to the axis of the solenoid.
As shown in Fig. 4, in the region between like-polarity poles (S-S or
N-N) of two adjacent repulsing magnets, 41 and 42, the transversal
magnetic field adds-up, while the axial magnetic field subtracts. As a
result, it can be shown that the (axial) magnetic field directed along the
ring of the rotor in the region between the S-S or N-N poles of the magnets
(and therefore in a direction along the solenoid axis) exhibits small
variations between two magnets. While the transversal magnetic field
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contributes to the mechanical traction/repulsion between the magnets and
the solenoid (due to Lorentz's force law) it does not contribute to the
magnetic flux through the solenoid. The (axial) magnetic field component,
directed along the axis of the solenoid, is the one that contributes to the
magnetic flux through the solenoid. However, the magnets of the rotor
move in a direction collinear to the solenoid axis, and the field component
in the collinear (axial) direction exhibits small variations in the region 43
between two repulsing magnets. It follows that there will be a small
change in magnetic flux during the transition, through the solenoid core,
of any rotor section located between any two magnets, and therefore the
back EMF generated during said transition will be small. Thus, if the
solenoids are energized with the proper timing sequence, the back EMF
effect opposing the DC supply voltage can be made small.
The invention will now be illustrated in detail with reference to an
illustrative preferred embodiment. As will become apparent from the
description to follow, the embodiment shown in the figures is only one of
many possible alternative systems and it has been chosen for this
description in view of its simplicity, it being understood that the invention
is by no means limited to said embodiment.
Reference is made to Fig. 5, which schematically shows a rotor
architecture that is suitable for a circular, rotating brushless DC motor
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according to one embodiment of the invention. The rotor consists of a
plurality of permanent magnets 51 (in the example of the figure, 5 of them
are shown) separated by a high permeability material 52, as may be, for
instance, iron. As is seen in the figure every two magnets 51 are separated
by a segment 52 made of highly permeable material, and all the magnets
and separating segments together form a ring-like structure. As explained
above, it is also possible to position the permanent magnets at fixed
distances between them along the circular path and to allow air to
separate them. Moreover, although it may be convenient to position the
magnets at identical distances between them, it may also be possible to
employ an asymmetric distribution of the magnets on the circumference
on which they are disposed.
Looking now at Fig. 6, a general, schematic view of the motor according to
this particular embodiment of the invention is seen in its assembled,
operating condition. Details of the various constructive elements of the
motor shown in this figure will be further illustrated with reference to
Figs. 7-14. As can be seen in the figure, the rotor 61 rotates inside a
plurality of solenoid assemblies 62. Power generated by the motor is
transferred out, according to this particular embodiment, using a gear 63.
The moment of the rotor is supported by bearings 64, which may be of any
suitable type. In the particular embodiment of this figure the assembly is
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positioned on a base 65. Also shown in the figure is toothed ring 101,
which will be further discussed with reference to Fig. 10.
Fig. 7 is a top view of the motor of Fig. 6, showing the same elements, and
Fig. 8 is a side view of the same motor. Fig. 9 shows the upper ring 91,
which is positioned above rotor 61, and illustrates its structural
relationship with bearings 64. The rings assembly associated with rotor 61
is shown in Fig. 10. According to the particular embodiment of the
invention shown in this figure a number of rings are associated with rotor
61. Upper ring 91, already described with reference to Fig. 9, is positioned
atop the rotor 61, and a rotor button ring 100 is positioned below the rotor
which rests on it. Below ring 100 toothed ring 101 is in geared cooperation
with gear 63 of Fig. 6. A bottom ring 102 is used to separate between base
65 of Fig. 6 and toothed ring 101. The assembly of the rings shown in Fig.
is connected together so that all rings rotate together and power is
transferred to gear 63.
The motor of Fig. 6 is shown in exploded view in Fig. 11, using the same
reference numbers as in the previous figures. As will be further discussed
with reference to Fig. 14, the solenoid housing 62 is conveniently made of
two pieces, to make the assembly possible.
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Fig. 12 further illustrates the motor of Fig. 6, with its central part being
shown in Fig. 12(a). Two cross-sections are further shown in the figure,
with Section A-A being an horizontal cross-section (Fig. 12 (b)), and
Section B-B being a vertical cross-section. Fig. 13 illustrates the
positioning of the permanent magnets 51 on ring 100. Fig. 13(b) is a cross-
section of ring 100 of Fig. 13(a), taken along the D-D plane, showing a
magnets 51 in place. According to this specific embodiment of the
invention, lower protrusion 131 of permanent magnets 51 fits its female
counterpart, i.e. groove 132 in lower ring 100, while its upper protrusion
133 fits a similar groove in upper ring 91 (not shown in the figure). Fig.
13(c) shows magnets 51 during its placement into the groove of ring 100,
and Fig. 13(d) is a perspective view of one such magnet already in place,
as in Fig. 13(b). The additional magnets, as well as the separating high
permeability material (52 of Fig. 5) are similarly located, and when all
elements have been placed and rings 91 and 100 are put in place, the rotor
assembly is ready to be positioned above toothed ring 101.
The various elements and their assembly is further illustrated in Fig. 14,
were the various parts are identified by the same numerals as in the
previous figures. Looking now at Figs. 3 and 6, to be understood that
solenoid housings 62 will be provided with a coil around them, which in
turn will be connected to a DC supply.
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Turning now to Fig. 15, an alternative rotor architecture is illustrated,
comprising a plurality of structures consisting of a magnet 151 which is
fixed on a base (not shown), and is flanked by a high-permeability
material 152, such as iron, on both sides. Gaps, indicated by arrow 153,
are left between each such two structures.
Fig. 16(a) is a schematic cross-sectional representation of the position of
the magnets within the solenoids, which minimize the EMF of the system.
In the figure two solenoids 161 and 162 are shown, with the South poles
163 and 164 positioned outside the solenoid. In this situation the EMF
may ideally reach down to zero, as illustrated in Fig. 16(b).
As will be appreciated by the skilled person the above description of one
specific embodiment of the invention is designed to illustrate the invention
in simple terms, but is not intended to limit the invention in any way.
Many modifications can be made to the motor of the invention. For
instance, the number of permanent magnets in the rotor can be increased
or decreased, many different mechanical arrangements can be provided in
order to transmit the power generated by the motor, and the gear shown
in the specific, illustrative embodiment described above is just one of
many alternative structures. Moreover, many different ways and schemes
for controlling the operation of the motor can be devised, including
controllers, software and sensors, all of which is within the scope of the
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skilled person and therefore has not been described hereinabove for the
sake of brevity.
